The Christian doctrine of the Trinity, and in particular the theology of "the consubstantiality of the Son with the Father," is crucial to Stephen's understanding of himself and his art. Both Telemachus and Proteus show him meditating on this relationship, and in Scylla and Charybdis he advances a reading of Shakespeare's Hamlet that relies upon what Haines calls “The Father and the Son idea. The Son striving to be atoned with the Father.” In this arcane personal mythology, Mulligan plays the part of a heretic, a modern representative of a "brood of mockers" who denied the mystery of the Trinity: Photius, Arius, Valentinus, Sabellius.

Earliest attested representation of the Scutum Fidei, in a detail from an illustration to the Compendium Historiae in Genealogia Christi by Peter of Poitiers (Cotton Faustina ms. B. VII, folio 42v), ca. 1210. Source: Wikimedia Commons. Other medieval manuscripts make clear that the links are non-directional, e.g. it is true not only that Pater non est Filius, but also that Filius non est Pater.

A modern version with the words written legibly. Source:Wikimedia Commons.

And the same, in English. Source: Wikimedia Commons.

Detail from an ilustration to the Summa Vitiorum or Treatise on the Vices by William Peraldus (Harleian ms. 3244, folios 27-28), showing a knight allegorically shielded from dangerous sins by the Scutum Fidei. Source: Wikimedia Commons.